interface.conf: Verify that an rtp-bridge interface is also bridged

RTP bridging only works when interfaces are bridged so we now check
that the interface has been configured as bridge upon seeing rtp-bridge
during parsing of interface.conf.

Note: This change requires that rtp-bridge appears after bridge for
the interface. If this makes your existing configuration fail because
rtp-bridge appears before the bridge parameter then please move the
rtp-bridge line below the bridge line.